FIG. 25.—Gravity filter block.
The earthy filter aids cannot be used with caustic or on some food products as they give an objectionable taste to them.
Kelly Filter.—This consists of a pressure tank mounted on a frame. A quick closing tank head is mounted on a carriage and track and this quick closing head and carriage support the filter leaves. The tank head contains all the
connections for introducing washes, compressed air, etc. The axis of the apparatus is slightly inclined to allow the filter leaves and filter head to run down and out of the pressure tank when the cake is to be discharged. The cycle of operations with the filter which requires usually about an hour beginning after a cake has been disposed of, is: (1) To run the leaves and head into the pressure tank and close it. (2) Material to be filtered is then introduced under any desired pressure and after filtration has begun it is only run fast enough to maintain any desired pressure until the cake of desired thickness has been accumulated. (3) The material in the tank at the time the cake has accumulated to the proper thickness is returned to the supply tank. (4) Washing is done under pressure. (5) The press is opened, the leaves are run to the lower end of the frame and the cake is discharged by compressed air.
Tanks can be built in a wide range of sizes. A tank 30 in. in diameter and 6 ft. long will have a net cubical capacity when the press is closed of about 25 ft. and a filter area of 140 sq. ft. A tank 6 ft. in diameter and 15 ft. long will have a net capacity of about 400 cu. ft. and a filter area of about 1,100 sq. ft. This represents about the standard range in size.1
Pressing.—Mechanically there are two principal types, the screw or hydraulic ram type and the worm type of press. The hydraulic presses are again divided into curb and plate presses. The curb presses consist of a cylindrical vertical bundle of slats or a cylinder of pierced metal which answers the same purpose, this is called the curb and is the material container. The liquid pressed out by the ram passes between the opening between the slats or through the holes in the metal cylinder. The curbs are hinged along a vertical line and open for removal of pressed material and for cleaning along a line facing the hinge. The slat curbs are held together with straps and are tightened after closing with cam locks. The curbs rest on the ram which is fashioned at its upper end into a saucer or gutter to receive the liquid which escape through the curb. The curt) with its content of material to be pressed is forced up against a circular plate which fits closely
1 For experimental -work on processes where a filter press will probably bo used in the final process and a small experimental filter press is not available, the effect may be gained by the use of a filter block as shown in the illustration. A wooden block from G to 14 in. square and 1 to 1 l/t in. thick is used. Grooves, A, are cut on the surface which connect with one long groove and thence by small holes, H in. to a central hole. A glass tube is placed in this, held by a short rubber tube to act as washer. The block is covered with filter cloth. A long piece of rubber tubing is then slipped over B and the